 <!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>自适应椭圆</title>
</head>
<style>
	.div{
		width: 400px;
		height: 200px;
		margin: 50px;
		background: green;
	}
	.circle1 {
		border-radius: 50%;
		/*border-radius: 50% / 50%; */
		/*border-radius: 200px / 100px;*/
	}
	/*半椭圆(纵轴对称)*/
	.circle2 {
		border-radius: 50% / 100% 100% 0 0;
	}
	/*半椭圆（横轴对称）*/
	.circle3 {
		border-radius: 100% 0 0 100% / 50%;
	}
	/*四分之一的椭圆*/
	.circle4 {
		border-radius: 100% 0 0 0;
	}

</style>
<body>
	<div class="div circle1"></div>
	<br>
	<div class="div circle2"></div>
	<br>
	<div class="div circle3"></div>
	<br>
	<div class="div circle4"></div>
</body>
</html>